1. Field of the Invention
The present invention relates to a light-emitting device including a light-emitting element utilizing electroluminescence (EL). Further, the present invention relates to a lighting device manufactured using the light-emitting device, or to an electronic device in which a display portion is mounted with the light-emitting device.
2. Description of the Related Art
In recent years, a light-emitting device to be mounted in a display of a television set, a mobile phone, a digital camera, or the like, or in a lighting device has been required to be flat and thin. A light-emitting device utilizing a self-luminous light-emitting element has attracted attention as a light-emitting device for meeting this requirement. One of the self-light-emitting elements is a light-emitting element utilizing electroluminescence, and this light-emitting element includes a light-emitting material interposed between a pair of electrodes and can provide light-emission from the light-emitting material by voltage application. Such a self-light-emitting element has a feature in that the thickness can be reduced and that response speed is extremely high.
In the next phase of this field, focus is placed on commercialization of a flexible light-emitting device having a curved surface, and a variety of proposals have been made (for example, see Patent Document 1). A light-emitting device using a flexible support substrate can be highly lightweight compared to the case of using a glass substrate or the like as a support substrate.
However, in commercialization of such a flexible light-emitting device, the biggest problem is its lifetime. The reason of the problem is that, in a flexible light-emitting device, a plastic substrate has to be used as a support substrate which needs to support a light-emitting element and to protect an element against moisture, oxygen, and the like in the outside thereof. The plastic substrate has high water permeability and low heat resistance, though it has flexibility. Because of the low heat resistance of the plastic substrate, a protective film with high quality which needs a high-temperature process cannot be formed, and moisture entering from the plastic substrate side has a great influence on the lifetime of the light-emitting element, furthermore, the light-emitting device.
In Non-Patent Document 1, for example, an example in which a light-emitting element is formed over a substrate including polyethersulfone (PES) as a base and is sealed with an aluminum film to form a flexible light-emitting device is introduced; however, its lifetime is about 230 hours and the light-emitting device is miles away from commercialization. In Non-Patent Documents 2 and 3, an example of a flexible light-emitting device in which a light-emitting element is formed over a stainless steel substrate is introduced. In this example, moisture and the like are prevented from entering from the stainless steel substrate side; however, moisture cannot be prevented effectively from entering from the light-emitting element side. Therefore, it is attempted to improve the lifetime with the use of, over the light-emitting element, a sealing film in which plural kinds of materials are stacked.
Although a thin metal film such as an aluminum film or a stainless steel substrate has both flexibility and low water permeability, it does not transmit visible light therethrough with a normal thickness. Thus, in the light-emitting device, a thin metal film or a stainless steel substrate is used for only one of a pair of substrates which sandwich a light-emitting element.